Jamming communications channels, a form of electronic-countermeasure (ECM), has been a basic tool of electronic warfare (EW) for decades. While jamming communications channels is effective at preventing unwanted communications, jamming also prevents or degrades desired communications within the jammed frequency spectrum.
There are several conventional jamming methods that attempt to prevent unwanted communications and allow desired communications. For example, time domain multiplexing may be used to prevent unwanted communications and allow desired communications by switching between jamming and communicating on a channel. A similar method shifts the jammed frequency band and the communications channel so that the two signals do not intersect in a frequency channel. A disadvantage of both of these methods is waiting for the RF energy to clear the desired channel space before the previously jammed channel can be used for communication.
Another method creates open channels through a jammed spectrum using double side-band suppressed carrier (DSBSC) mixing of the jamming signal. DSBSC mixing jams a very wide spectrum of frequencies leaving open channels at the frequency carriers used for signal conversion. For example, jamming a signal spectrum between 1 MHz and 181 MHz having a clear channel at 91 MHz may be achieved by mixing a jamming signal between 1 MHz and 90 MHz with a carrier signal at 91 MHz. This provides a clear channel spacing of 2 MHz. However, there are major drawbacks to such a system.
For example, a DSBSC mixing system has difficulty controlling channel spacing and roll-off. In addition, the channel spacing may be extremely wide when compared to the transmission signal bandwidth that is actually needed to guarantee acceptable channel characteristics. Furthermore, the spacing of multiple channels depends on the selection of carrier frequencies and the ability to band limit the jamming signal to accommodate the communication channel selection. The channel separation also must be greater than the bandwidth of the jamming spectrum that is converted by a mixer. As a result, the DSBSC system requires a complex array of adjustable bandpass filters or strict limitations on channel spacing and location.
Yet another method is to “burn” through the jamming signal by overpowering the jamming signal. However, this approach is limited to systems with significant power output capabilities and the quality of the transmission may still suffer appreciably.